I have only ever been on one cruise, I went on one of the Hurtigruten (spelling?) cruise up and down the Norwegian coast to see very elusive northern lights. The boats also act as ferries for local people, goods etc so stop off frequently, even during the night, to small communities, so I suppose they do serve a social purpose as well as a tourist one , though not sure that justifies their existence. I was naively quite impressed with the size of the boat, until we drew up alongside one of the mega cruise ships and I was astounded.
My experience was that while I enjoyed the experience as a one off there is a certain type of person who likes cruises, and it isn't me. I really can't see the point of the ones where they descend on a small port and thousands of people disembark for a couple of hours , take million photos, buy crap souvenirs then tick Venice , for example, off their bucket list.
(Though I did enjoy sitting alone in the sauna, looking out through a wall sized window as we sailed through frozen sea)